Climate Extremes – Why the past is just as important as the future

Figure 1: Loire River near Ancenis, France, during a drought (Stephane Mahe/Reuters).

Climate change is a major threat to humanity, on a health, economic and social level. Large-scale mean changes, such as increasing global mean temperatures, will often lead to more severe and frequent regional extremes. Events like the 2003 European heat wave, which caused tens of thousands of deaths across the continent, will become more likely in a 2 ºC warmer world. Coping with these changes requires a robust understanding of how climate extremes will change in the future. Luckily, the past climate can provide us with many valuable clues of what lies ahead.

What lies ahead

Considerable effort has gone into developing projections of future extreme events using climate models. However, due to the rare occurrence of extremes, the confidence in these projections is often quite low. Nonetheless, there is evidence that many regions of the globe will witness an increase in heavy precipitation events, hot days and prolonged droughts [1]. An example of this is illustrated in figure 2, which shows the number of local monthly record-breaking temperatures in our changing climate, relative to those expected in a climate without warming.

Figure 2: Ratio between local monthly record-breaking temperatures in our climate and the number expected in a world without climate change. The blue line shows the output of a statistical model based on the change in global mean temperatures. The light blue shading gives the uncertainty range. The orange and red lines correspond to annual and 5-yearly global data, respectively [2].

For example, a ratio of 10 indicates that our climate has ten times as many local extremes as one which doesn’t warm. The blue line shows the output of a statistical model, based on the change in global mean temperatures. The faint and dark red lines correspond to annual and 5-yearly global data, respectively. In recent years, we have already experienced as many as thirteen times more record-breaking high temperatures than a century ago. An accelerating upward trend is evident in all three lines [2].

Past and Future

In order to provide a realistic picture of what lies ahead, it is important to achieve a robust scientific understanding of the climate mechanisms driving extreme events. To gain this understanding, the extremes must be placed in the context of large-scale atmospheric circulation and long-term trends and variability. Specifically designed model simulations and data-based reconstructions of the climate of the past provide the means to do this. This is why studying the past is just as valuable as forecasting the future.

A very good example of using the past to understand the future is provided by the effects of the North Atlantic Oscillation (NAO) on European winters. The NAO index measures the pressure difference between Iceland and the Azorres islands, and its sign is known to be linked to specific weather patterns over Europe and the eastern portion of North America. The index has seen a significant increase in the latter part of the 20th century, which the majority of climate models have struggled to simulate. There are, however, some models which do realistically reproduce the changes in the NAO. These suggest that long-term trends in the NAO can explain a large part of the changes in extreme wintertime temperatures and precipitation witnessed in Europe between the 1960s and the 1990s. In turn, this implies that if models do not capture the long-term variability of the NAO, this can significantly affect their projections of the future. Studying the NAO in the last century therefore highlights a potential shortcoming of our forecasting tools. It also indicates that we might have underestimated future changes in climate extremes over Europe [3].

Another example of the close link between the climates of the past and of the future is provided by datasets reconstructing climate variability over the last hundreds, or even thousands, of years. Proxy data, combined with documentary evidence, can provide valuable indications of the range of climate variability and the frequency of climate extremes well before any systematic weather data was collected. The datasets can then be used to constrain long-term model simulations, and assess whether there are inconsistencies or shortcomings in the model output. Examples range from evaluating the effect of volcanic aerosols on surface temperatures to looking at anthropogenic influence on the climate of the central Pacific [4].

Due to the major impacts extreme events in a changing climate are likely to have on humanity, it is crucial that we develop the best possible forecast tools to probe what lies ahead. In order to do this, understanding the past is as important as looking to the future. The development of new datasets, and the design of model simulations aimed at uncovering the mechanisms which have driven past extreme events, are an essential part of the effort to predict the future of our planet.

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